Chemical ionization involves the transfer of charged species from reagent ions to analyte molecules to produce analyte ions that can be subsequently mass analyzed. The charged species most commonly formed in positive ion mode is the adduct between an analyte molecule and positive hydrogen ions (H+).
Chemical ionization conducted at atmospheric pressure is commonly known as Atmospheric Pressure Chemical Ionization (“APCI”). A sample containing analyte material is typically delivered to an Atmospheric Pressure Chemical Ionization ion source as a solution. The solution containing the analyte is then sprayed into a heated tube through which a nebulising gas is also directed. The nebulising gas causes the solution to be finely nebulised into fine droplets which then impact the inner wall of the heated tube and are converted into the gas phase. As the solution is converted into the gas phase the analyte molecules become desolvated. Hot gas comprising mobile phase, microdroplets and desolvated analyte molecules then exit the heated tube and the analyte molecules are then subsequently ionized by chemical ionization with reagent ions. In particular, analyte molecules are ionized by gas phase ion-molecule reactions between reagent ions and the analyte molecules.
Reagent ions which transfer charged species to the analyte molecules to form analyte ions are produced as a result of a corona discharge in the solvent vapour. The corona discharge is generated by applying a high voltage (e.g. 5 kV) to the tip of a sharp corona needle or pin. Analyte molecules are ionized by gas phase ion-molecule reactions in the corona discharge region located between the corona tip and the ion sampling orifice. Analyte ions are therefore generated in the region of the corona discharge since this is where the reagent ions are formed.
A majority of the gas exits the ion source via an exhaust port whilst a small proportion of the gas and analyte ions are drawn through an ion sampling orifice into the vacuum system of the mass spectrometer for subsequent mass analysis.
Conventional APCI ion sources may be considered to have relatively open geometry design. The corona needle is located at the outlet of the heated tube but gas and molecules exiting the heated tube are not constrained to remain in the region of the corona needle. The corona needle is also located adjacent to the ion sampling orifice of the mass spectrometer. The corona needle, heated tube and ion sampling orifice may be enclosed in a large volume enclosure but the large volume enclosure itself has little impact upon the ionisation process.
APCI is in theory applicable to a wide range of analyte polarities. However, highly polar or ionic analytes are more commonly analysed by Electrospray Ionisation (“ESI”) ion sources than by APCI ion sources since a problem with conventional APCI ion sources is that when they are used to ionise highly polar analyte molecules the ion signal is observed to decrease as the corona voltage or current is increased. Low to moderately polar analyte samples are more commonly analyzed by APCI ion sources since such analytes exhibit the opposite effect to highly polar analytes i.e. the ion signal intensity increases as the corona voltage or current is increased. Therefore, although conventional APCI ion sources are suited to ionising low to moderately polar analytes they are not best suited to ionising highly polar analytes.
Another particular problem with conventional APCI ion sources is that they are also not particularly suited to analysing a mixture containing both low to moderately polar analytes and also highly polar analytes since the different types of analytes require different corona current settings for optimal ionisation. When faced with the task of using a conventional APCI ion source to ionise a sample comprising a mixture of both low to moderately polar analytes and also highly polar analytes, the conventional approach is either to set the corona current at a compromise setting or more commonly to perform two separate experimental runs or acquisitions. If two separate experimental runs or acquisitions are performed then in the first experimental run or acquisition the corona current may, for example, be optimized for low to moderately polar analyte ions and in the second subsequent experimental run or acquisition the corona current may be optimized for highly polar analyte ions (or vice versa). As will be appreciated such an approach therefore results in a doubling of the total analysis time and also a doubling of the total sample consumption. Both of these increases are problematic. The increase in sample consumption is particularly problematic if only a very small amount of sample is available for analysis (which is often the case).
It is therefore desired to provide an improved ion source. In particular it is desired to provide an APCI ion source which can efficiently analyse a sample comprising both low to moderately polar ions and also highly polar ions.